1. Field of the Invention
This invention relates to a variable magnification optical apparatus, and in particular, to an optical apparatus in which an image of an object is formed with a magnification different from a predetermined magnification to thereby perform various measurements of the object to be inspected.
2. Description of Related Art
A variety of variable magnification optical apparatuses have been proposed in the past. Japanese Utility Model Publication No. Hei 4-3291, for example, is directed to a variable magnification optical system which does not cause the shift of an optical axis, namely optical misalignment, by a change of magnification, and discloses a variable magnification optical apparatus equipped with a fixed objective lens, two relay lenses of different magnifications, an imaging plane, and shutters for selectively blocking two optical paths.
Japanese Patent Preliminary Publication No. Hei 2-281223, for the purpose of facilitating a change of magnification for observation without replacing an objective lens, discloses an observation optical apparatus equipped with a fixed objective system of a constant magnification and a variable magnification optical system for changing the magnification of an observation image formed by the objective system to a predetermined magnitude.
Japanese Patent Preliminary Publication No. Sho 64-28611, for the purpose of keeping the alignment of an optical system even when a magnification is changed, discloses a variable magnification optical apparatus having an objective lens, two relay lenses of different magnifications, a member for making the optical axes of the two relay lenses coincide with the position of an image, and insertable blocking members for blocking two optical paths.
Further, Japanese Patent Preliminary Publication No. Hei 5-127096 refers to an ultraviolet microscope and discloses a technique for separating an optical image of an object formed by an objective lens into two, an ultraviolet image and a visible image, and providing the first observation means for observing the ultraviolet image and the second observation means for observing the visible image so that the variable magnification lens systems of the first and second observation means can be set to different magnifications.
In general, an objective lens is such that its numerical aperture (NA) increases with increasing magnification to enhance a resolving power. The above-mentioned prior art relative to variable magnification systems does not refer to the NA in connection with the magnitude of magnification in observation. It is therefore considered that each of the conventional variable magnification optical systems used the same NA, notably a small NA, at high and low magnifications. This is because if a large NA is set and a wide field is observed at a low magnification, the large NA can be obtained at the center of the visual field, but not in the periphery thereof, with the result that the amount of marginal light becomes insufficient. Thus, to avoid this difficulty, a smaller NA must be set.
Where the visual field is observed at a low magnification, a small NA does not cause any problem, while at a high magnification, the problem is produced that if the NA is small, resolving power becomes insufficient.